Maximum likelihood based interference cancellation for space-time coded signals

ABSTRACT

A signal processing apparatus comprises a plurality of receiving elements, weighting means, and decoding means. Each of the receiving elements has respective weighting means associated therewith at the decoding means, and is arranged to receive a plurality of signals transmitted from a plurality of transmitters. The weighting means are arranged to apply a complex weighting function to each of a number of the signals received by the receiving elements at a given frequency in order to null the number of signals. The decoding means are arranged to determine a symbol or codeword associated with a non-nulled signal and to incorporate the symbol or codeword in the determination of at least one further symbol or codeword.

This invention relates to a signal processing method and apparatus. Moreparticularly, but not exclusively, it relates to a method and apparatusfor decoding symbols and/or codewords in a multiple input-multipleoutput orthogonal frequency division multiplex wireless network system.

It is known to convert a stream of serial data symbols to a parallelblock of data symbols and transmit each of the symbols via a respectiveone of a number of transmitters in order to increase data transfer rateacross a wireless network, for example the BLAST architecture.Space-time coding improves the quality of data links between transmitterand receiver by increasing redundancy in codewords transmitted comparedto the input symbols and by producing multiple codewords withsignificant differences therebetween for each frame of symbols coded.

Space-time trellis codes (STTC) bridge the divide between the twoabove-mentioned techniques wherein individual parallelised data streamsare protected by space-time codes. This yields improved performance interms of the robustness of communications and therefore gives animproved veracity of data .associated with space-time codes. Such asystem is particularly attractive for multiple input-multiple outputsystems that have a large number of antenna elements, for example fouror more transmitters and four or more receivers. This is due to thesystem allowing high data transfer rates with high confidence in datatransfer quality.

Upon receiving a corrupted codeword an ideal STTC receiver performs asearch over all possible codewords and chooses a vector or symbols thatmaximises a likelihood function, for example using a Viterbi decoder oncodewords received by a number of antennas. However, the complexity ofthe solution of such a likelihood function increases exponentially withthe number of transmitter elements. Also, increasingly complexmodulation schemes can be employed, for example 64 quadrature amplitudesmodulation (QAM). In 64-QAM there are sixty-four distinct symbols invector space, each one representing a six bit binary sequence. This tooadds to the complexity of solving the vector Viterbi equation. Thus, acomputational solution to the problem is prohibitively computationallycomplex to implement practically.

One approach to reduce the computational complexity of solving a maximumlikelihood equation is to employ group interference suppression (GIS).In GIS spatial suppression of codewords originating from undesiredtransmitters occurs at the receivers leaving only a codeword from adesired transmitter to be detected. This is achieved by weightingsignals received at the receivers in order to produce a zero, or nearzero, signal in a given direction, a process known as “nulling”. It willbe appreciated that the terms direction may not be a physical directionin the case of an indoor or multipath rich environment.

Once the desired codeword is determined the entire nulling process hasto be repeated for the next codeword. This is clearly a cumbersome andtime consuming process.

In orthogonal frequency division multiplexing (OFDM) as shown in FIG. 2a number of sub-carrier frequencies, each being a harmonic of afundamental, lowest frequency, sub-carrier, have complex data valuesimposed upon them by use of an inverse fast Fourier transform (IFFT)unit. The complex data values vary the phase and amplitude of thesub-carriers away from their unperturbed state. Upon transmission, thesub-carriers superpose to produce a non-sinusoidal signal. Uponreception of the signal by a receiver a fast Fourier transform (FFT) iscarried out upon the signal to recover the sub-carriers and hence theirassociated data values. A typical wireless local area network (WLAN)data transmission will include sixty-four sub-carriers, for exampleHiperlan 2 and IEEE 802.11a.

In the case of an OFDM STTC system each sub-carrier frequency from onetransmitter will interfere with the same sub-carrier frequencyoriginating from any other transmitter. There will however be nointerference between sub-carriers of different frequencies when thecyclic prefix is at least as long as the channel excess delay. Thus, inan OFDM STTC system the complexity of applying a conventional GISsolution increases many fold, as spatial nulling has to be performed notonly for each receiver element but also at each individual sub-carrierfrequency. This substantially increases the complexity of implementationof such a system and increases the computational complexity of the GIStechnique still further.

It is an object of the present invention to provide a method of signalprocessing that, at least partially, ameliorates at least one of theabove-mentioned problems and/or disadvantages.

It is a further object of the present invention to provide a signalprocessing apparatus that, at least partially, ameliorates at least oneof the above-mentioned problems and/or disadvantages.

According to a first aspect of the present invention there is provided amethod of determining each of a plurality of data symbols or codewordsfrom a plurality of signals comprising the steps of:

(i) weighting a number of said signals so as to substantially null saidnumber of signals, using weighting means;

(ii) determining a symbol or codeword associated with a, or each,non-nulled signal using processing means arranged to execute a maximumlikelihood estimation process upon said, or each, non-nulled signal;

(iii) reducing the number of signals nulled by the weighting means by atleast a number of non-nulled signals in step (ii);

(iv) altering the maximum likelihood metric in accordance with the datasymbol derived at step (ii); and

(v) repeating steps (ii) to (iv).

This method has the advantage over prior art methods that assuccessively fewer input sources are suppressed, nulled, at eachiteration of the method the number of spatial degrees of freedomavailable for sampling increases due to step (iv). This increasesdiversity on receive for sampling, which in turn increases theconfidence with which later data symbols can be determined.

The method may include sampling data, typically channel stateinformation (CSI), to determine which signals are to be nulled ateither, or both, of steps (i) and (iii). The method may includeselecting signals with lowest input power to be nulled at either, orboth, of steps (i) and (iii). This has the advantage that those signalswith the highest input power have their associated data symbolsdetermined first. This is important as high input powers are usuallyassociated with high signal to noise ratios and thus confidence in theinitially determined symbols is increased. High confidence in theinitially determined symbols is important as any subsequentdetermination of further symbols employs these initial symbols anderrors propagate in such a trellis-coded system.

The method may include determining symbols that are, or form part of,codewords, the codewords typically being associated with streams ofsymbols input to a transmitting means.

The method may include separating frequencies of at least some of theplurality of signals by multiples of a harmonic frequency. The methodmay include orthogonalising the plurality of signals. Thus, the methodis directly applicable to orthogonal frequency division multiplexing(OFDM) of signals.

The method may include providing a plurality of receivers arranged toreceive said plurality of signals prior to step (i). The method mayinclude transmitting a signal that is a sum of said signals from aplurality of spatially separated transmitters.

The method may include deriving a matrix of complex weightingco-efficients by the processing means to be applied to said weightingmeans in order to null said signals at either of steps (i) or (ii).

The method may also include applying said weighting coefficients to saidweighting means.

The method may include using the vector Viterbi algorithm at step (ii).

The method may include parallelising an input serial stream of datasymbols prior to transmission.

The method may include coding a frame of parallelised data symbolstypically using space-time coding prior to transmission.

The method may include coding a frame of 2^(n) parallelised data symbolsprior to transmission, where n is an integer selected from the followinglist: 1, 2, 3, 4, 5, 6, 7, 8, 16, 32, 64, 128, >128.

The method may include producing at least one codeword, preferably two,during the coding operation.

The method may include reducing the number of nulled channels betweensteps (ii) and (iv).

The method may include increasing the diversity upon receive of theplurality of signals.

According to a second aspect of the present invention there is provideda signal receiving apparatus comprising a plurality of receivingelements, weighting means, and decoding means, each of the receivingelements having respective weighting means associated therewith, each ofthe receiving elements being arranged to receive a plurality of signalstransmitted from a plurality of transmitters, the weighting means beingarranged to apply a complex weighting function to each of a number ofsaid signals received by the receiving elements at a given frequency inorder to null said number of said signals, the decoding means beingarranged to determine a symbol or codeword associated with a non-nulledsignal and to incorporate said symbol or codeword in the determinationof at least one further symbol or codeword.

The receiving apparatus may include at least four receiving elements.

Each receiving element may have a channel state information (CSI) unitassociated therewith, and each CSI unit may be arranged to compensatefor distortion to the signal received by the apparatus due to variationsin the transmission path of said signal.

The receiving apparatus may include an FFT unit between each receivingelement and the decoding means, and the FFT units may be arranged toseparate each of a plurality of sub-carrier signals from said receivedsignals.

The decoding means may include processing means arranged to carry out amaximum likelihood estimation procedure upon a sub-carrier signalreceived at a receiving element in order to determine the symbol.

The processing means may be arranged to carry out whole vector Viterbidecoding upon the signal.

The apparatus is preferably arranged to execute a method in accordancewith the first aspect of the present invention.

According to a third aspect of the present invention there is provided amethod of increasing data transfer capacity across a network comprisingthe steps of:

(i) receiving a signal comprising a plurality of data carryingsub-channels transmitted by a plurality of transmitter elements at aplurality of receiving elements;

(ii) suppressing a component of the signal associated with a givensub-channel transmitted by all but one transmitting element;

(iii) determining a symbol or codeword associated with said signalreceived on said given sub-channel at said one receiving element using amaximum likelihood estimation process; and

(iv) incorporating the symbol or codeword into the maximum likelihoodestimation process for the determination of at least one other symbol orcodeword.

The method may include parallelising data and encoding the data as asymbol or a space time codeword prior to transmission of the symbol orcodeword over the network.

The method may include providing more than four receiving elementsarranged to receive the signal from the network, and the method mayinclude providing more than four transmission elements arranged totransmit the signal over the network.

The method may include applying a whole vector Viterbi decoding to thesignal at step (iii).

The method may include providing the network in the form of a wirelesslocal area network (WLAN), for example IEEE802.11a, HiperLan 2 orBluetooth networks or a telecommunications network. It will beappreciated that in the case of current narrow band Bluetooth networksspace-time trellis coding is applied.

According to a fourth aspect of the present invention there is provideda computer readable medium having stored therein instructions forcausing a device to execute the method according to either of first orthird aspects of the present invention.

According to a fifth aspect of the present invention there is provided aprogram storage device readable by a machine and encoding a program ofinstructions which when operated upon the machine cause the machine tooperate as the apparatus in accordance with the second aspect of thepresent invention.

The invention will now be further described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a representation of a quaternary phase shift keying (QPSK)signalling scheme of the prior art;

FIG. 2 is a representation of an orthogonal frequency divisionmultiplexing (OFDM) modulation scheme of the prior art;

FIG. 3 is a schematic representation of generation of codewords from aserial input streams of data, of the prior art;

FIG. 4 is a schematic representation of a spatial nulling scheme of aphased array antenna of the prior art;

FIG. 5 is a representation of possible symbols and a data vectoraccording to both the prior art and the present invention;

FIG. 6 is a schematic representation of a wireless multipleinput-multiple output (MIMO) space-time trellis coding (STTC) systemaccording to at least an aspect of the present invention;

FIGS. 7 a to 7 d are graphs showing an improvement in performance of areceiver according to an aspect of the present invention compared to aprior art receiver;

FIG. 8 is a flow diagram detailing a method of signal processing inaccordance with to the first aspect of the present invention; and

FIG. 9 is a flow diagram detailing a method of signal processing inaccordance with the third aspect of the present invention.

Referring now to FIG. 1, a constellation 100 of a known quaternary phaseshift keying (QPSK) signalling scheme comprises four symbols 102 a-dspaced apart in the complex plane. The relative complex and realcomponents of the symbols 102 a-d denote which two bit binary sequenceis represented by a given symbol, for example positive real and positiveimaginary components, symbol 102 a represent the two bit binary sequence00. Thus it is possible to represent two bits using a single symbolusing QPSK, effectively doubling the bit rate over direct binarysignalling.

Referring now to FIG. 2, an orthogonal frequency division multiplexingarrangement 200 comprises four input channels 202 a-d, an inverse fastFourier transform (IFFT) unit 204, and a fast Fourier transform unit 208(FFT).

The IFFT units 204 generate, in this case, four sinusoidal sub-carriers210 a-d. The first sub-carrier 210 a has a frequency and constitutes afundamental of the system. Each of the other three sub-carriers 210 b-dhave frequencies that are multiples of the frequency of the fundamental210 a, that is to say that they are harmonics of the fundamental 210 a.

The input channels 202 a-d carry digitised data to the IFFT unit 204where the data is converted to a complex value. Each complex value isapplied to a respective sub-carrier 210 a-d. This has the effect ofvarying the phase and amplitude of the respective sinusoidalsub-carriers 210 a-d.

The sub-carriers 210 a-d are combined to form a non-sinusoidal carrierwave 212. The carrier wave 212 is transmitted to a receiver where theFFT unit 208 separates out the sub-carriers 210 a-d and extracts thecomplex weightings from them. These complex weightings are then decodedto recover the strings of data applied at the input channels 202 a-d.

Referring now to FIG. 3, an encoder 300 comprises a serial to parallelconverter 302 and a codeword generator 304. In use, a serial string ofdata 306 comprising a plurality of data blocks 308 a-f spaced apart intime, is input into the convertor 302. The blocks 308 a-f are outputfrom the convertor 302 at a plurality of output channels 310 a-fsimultaneously. This allows either the discrete output of data from asingle data block of a serial data stream or alternatively theconstruction of multiple serial frames from data blocks spaced apartwithin the initial serial data stream 306. For example, in this caseevery seventh block of data will be placed adjacent each other in a newframe 312 a-f. These frames can be of indeterminate or user definedlengths.

Each of the frames 312 a-f is coded into two codewords 314 a, b by thegenerator 304. The two codewords 314 a, b have a high degree ofredundancy and have differences between them maximised. This results insmall variations in the input frames 312 a-f giving large differencesbetween codewords, for example a one bit difference between two framescan result in the variations of four or five symbols in the codewordsgenerated by the generator 304. Also, the coding process builds in anelement of memory to the codewords, in that the codewords generatedyield information about data within a given frame.

It is these codewords that are transmitted via the sub-channels in anOFDM system. In a multiple input-multiple, output (MIMO) OFDM systemeach codeword is sent to a respective transmission antenna, typicallytwo or more antennas as this increases spatial diversity on transmit.

The reception antennas can be arranged to co-operate in order tospatially reject transmitted signals, by group interference suppression(GIS), this is shown in FIG. 4.

Referring now to FIG. 4, this shows the case for a plane wave incidentupon a detector array two receiving elements 402 a, b define an arrayaperture 404. A wavefront 406 is incident upon the aperture 404, at anangle θ to the normal of the aperture 404, along a vector A-A.Considering the two receiving elements 402 a, b, the wavefront 406 musttravel an additional distance y after being received by the element 402a before being received by the element 402 b. From a simple geometricconsideration it can be seen that y=d sin θ. This extra distance oftravel introduces a phaseshift between the wavefront received at the twoelements 402 a, b, of: $\begin{matrix}{\Phi = {\left( \frac{2\pi}{\lambda} \right)d\quad{Sin}\quad\theta}} & \left( {{Equation}\quad 1} \right)\end{matrix}$Weighting units 408 a, b apply a correction in order that the electricvectors of the respective fractions of the wavefront 406 detected at thereceiving elements 402 a, b are aligned prior to exiting thisarrangement. Thus it can be seen that by altering weightings applied tothe fractions of the wavefront 406 at the weighting units 408 a, b theantenna array can be spatially scanned as each directions will exhibit aunique phase relationship between the receiving elements 402 a, b.

It will be appreciated that the present invention is not limited to aplane wave situation and the above description should not be taken aslimiting.

Referring now to FIG. 5, a transmitted codeword, represented as a vector502, is placed in a Cartesian framework. Any one of a number of possiblesymbols 504 a-d within the signalling scheme could correspond to thetransmitted symbol upon reception, and in order to determine betweenthem a measure must be made of the straight line distance between theterminal points of the vectors representing the codewords. The shortestEuclidean distance will constitute the best fit between the transmittedsymbol upon reception and the allowable symbols within the signallingscheme.

Referring now to FIG. 6, a wireless MIMO OFDM network 600, for example awireless local area network (WLAN) or mobile telecommunications network,comprises a transmitter unit 602 and a receiver unit 604.

The transmitter unit 602 comprises a serial to parallel converter 606,typically a BLAST architecture, a frequency space encoder 608 a-n, aplurality of IFFT units 610 a-n and a plurality of transmit antenna 612a-n. Each antenna 612 a-n is connected to a respective IFFT unit 610a-n.

The receiver unit 604 comprises a plurality of receive antenna 614 a-heach of which is connected to a respective FFT unit 616 a-h, a decoder618, including weighting units 619 a-h for GIS, and a plurality channelstate information (CSI) modules 620 a, h. Each CSI module 620 a-h (only2 shown) is associated with a respective FFT unit 616 a-h and makes anestimation of the distortion to the received signals on each sub-carrierdue to the path travelled by the received signal, for example, byreflections of the signal from surfaces in the transmission path etc.The CSI also corrects for this at the receiving unit 604 in order torecover the transmitted symbols.

The FFT units 616 a-n separate out sub-carriers from a non-sinusoidalcarrier wave. The sub-carriers are passed from the FFT units 616 a-n tothe decoder 618 where GIS is carried out, using the weighting units 619a-h. Frequency-space vector Viterbi decoding is also carried out on thesub-carriers at the decoder 618 such that symbols transmitted fromthe-transmission unit 602 can be recovered.

In a MIMO OFDM network sub-carriers with the same frequency willinterfere with each other irrespective of which transmit antenna 612 a-nthey originate from. This necessitates signal processing at the receiver604 in order to correctly decode codewords transmitted from transmittingantennas 612 a-n, and hence recover transmitted data. In OFDM coding ofsymbols takes place across sub-carrier domains decoding takes place inboth spatial and frequency (sub-carrier) domains. Thus, each transmitset of antennas 612 a-n is suppressed in turn, using GIS, in order tonull all but one group of transmitting antennas, decode the codeword andmodify metrics used in maximum likelihood decoding of subsequentcodewords.

This leads to linear equations to be solved where:

G is the number of transmit antenna groups;

N_(t) ^(g) is the number of transmit antennas in the g th group(typically 2), where 1<g<G;

c _(k)=(c¹ _(k), c² _(k), . . . c^(g) _(k))^(T) is the space-frequencysymbol transmitted on the κ^(th) subcarrier frequency.

For example, if the g th group has 2 transmit antennas within it thenc^(g) _(k) will actually consist of two symbols transmittedsimultaneously from the transmit antennas. The symbols are part of thecodeword that encompasses all the sub-carriers.r _(k) =H _(k) ·{overscore (c)} _(k)+η_(k)   (Equation 2)

r_(k) is the received signal at the κ^(th) subcarrier frequency for alltransmit antennas;

H is a matrix of the CSI for each sub-carrier;

η_(k) is the additive white Gaussian noise at the receivers

In order for a given receiver to decode the g th codeword the receiversuppresses space-frequency codewords originating from other groups oftransmitters according to the following:θ_(k)(c _(k) ^(g))r _(k)=θ_(k)(c _(k) ^(g))H _(k){overscore(c)}_(k)+θ_(k)(c _(k) ^(g))η_(k)   (Equation 3)Where:

θ_(K) (c^(g) _(k)) is an orthonormal basis set for null space of amatrix composed of all columns of H_(k) except for those correspondingto the transmit antennas forming the g th group whose codeword it isdesired to decode.

The decoder 618 takes the signal received at the antennas 614 a-n, andcorrected for the signal path by the CSI units 620 a-n, and executeswhole vector Viterbi decoding upon the signal. The maximum likelihood(ML) codeword transmitted from the g th group of antennas is given by:$\begin{matrix}{\overset{\sim}{c} = {\underset{({00_{c}^{g}00})}{\arg\quad\min}{\sum\limits_{k = 0}^{K - 1}{{{{\theta_{k}\left( c_{k}^{g} \right)}r_{k}} - {{\theta_{k}\left( c_{k}^{g} \right)}H_{k}{\overset{\sim}{c}}_{k}}}}^{2}}}} & \left( {{Equation}\quad 4} \right)\end{matrix}$Where:

{tilde over (c)} is the decoded codeword

K is the total number of sub-carriers used

Equation 4 describes the process of searching over all possiblespace-frequency codewords that could have been sent by the g th transmitantenna group and selecting the most likely codeword given thatcodewords from all other groups of transmit antennas have been excludedvia the GIS procedure. Once the a codeword is decoded the whole processis repeated taking into account the decoding of the decoded codeword,according to the following: $\begin{matrix}{\overset{\sim}{c} = {\underset{({00{\overset{\sim}{c}}^{g}0c^{g^{\prime}}})}{\arg\quad\min}{\sum\limits_{k = 0}^{K - 1}{{{{\theta_{k}^{\prime}\left( c_{k}^{g^{\prime}} \right)}r_{k}} - {{\theta_{k}^{\prime}\left( c_{k}^{g^{\prime}} \right)}H_{k}{\overset{\sim}{c}}_{k}^{10}}}}^{2}}}} & \left( {{Equation}\quad 5} \right)\end{matrix}$Where θ′_(k)(c_(k) ^(g′)) is an orthonormal basis for the null space ofall columns of H_(k) except those corresponding to the transmit antennasof the current group g′ being decoded and all the transmit antennas ofpreviously decoded codeword groups.

Any previously decoded codeword is accounted for by modifying the MLmetric such that any previously decoded codeword no longer need besuppressed through the GIS process. Thus, on each iteration the numberof constraints on the GIS nulling matrix is reduced and hence a largernumber of degrees of freedom available on the received signal. Thisleads to greater spatial diversity and improved performance in terms ofrobustness of communications.

In prior art arrangements search space would have to be reformulatedusing GIS in order to determine each codeword. This is no longer truemaximum likelihood as the search space is artificially restricted byGIS. Such an approach is sub-optimum.

In a detection scheme in accordance with the present invention thedetector 618 determines which of the received codewords has the greatestsignal strengths typically by use of a comparator that compares thepower, calculated from each CSI vector, in order to determine therelative power of each channel, and hence signal. The detector 618 thusdetermines the most likely codeword using equations 1 and 3 ashereinbefore described.

This scheme has the effect of changing the mean of the noise vector ofthe system to become the first detected codeword. Thus, the firstcodeword is ignored during the GIS procedure in determining the secondcode. This allows more degrees of freedom, by freeing receiving antennasto be used in the detection of lower power signals, thereby increasingthe signal to noise ratio of such signals due to the inherent increasein receive diversity.

The first detected space time stream is explicitly incorporated into thebranch metrics of the detection of the second code.

At each detection step an additional degree of freedom is introducedinto the receive diversity as an additional receiving antenna orantennas 614 a-n can be used for reception of a codeword. This isbecause at each detection step the previously detected codeword isignored by the GIS procedure.

Assuming that the first, and subsequent, codewords are detectedperfectly there is no impact on the detection of subsequent codewords bythe Viterbi detector 618.

Referring now to FIGS. 7 a-d, the modelled performance of a system inaccordance with the present invention is shown in comparison to theperformance of a conventional frequency space coded (FSC)-BLAST-OFDMsystem using two transmit and four receiving antennas as shown in FIG.6. FIGS. 7 a and 7 b being from a first decoding group (first twoantennas) and FIGS. 7 c and 7 d being from a second decoding group. Thesystem modelled has four codewords with two transmit antenna each, fourreceive antenna and assumed ideal channel state information, i.e. nodistortion to the transmitted codes upon reception.

As can be seen from FIGS. 7 a and 7 c the bit error rate (BER)associated with the present invention is approximately 1.5-2 dB improvedover the conventional FSC-BLAST-OFDM architecture at a given signal tonoise ratio (SNR). This is however of only limited importance as thedata transmitted over the network will usually be in the form of frames,typically fifty-four bytes in length. If an error is found in a framethe whole frame must be re-transmitted. Therefore what is of moreinterest than the BER is the frame error rate (FER). This is shown inFIGS. 7 b and 7 d and improvement of 1.5 to 2 dB in the FER overconventional FSC-BLAST-OFDM architectures is observed at a given SNR foran arrangement in accordance with the present invention.

Referring now to FIG. 8, a method of signal processing that increasesdiversity on receive for sampling, thereby increasing the confidencewith which later data symbols can be determined, comprises weighting anumber of received signals in order to substantially null them (Step800). A data symbol or codeword associated a non-nulled signal isdetermined using signal processing means that are arranged to execute amaximum likelihood estimation process upon the non-nulled signal (Step802).

The number of signals nulled by the weighting means is reduced by one,i.e. the signal for which the symbol or codeword had been determined(Step 804). The symbol determined is included in the maximum likelihoodestimation procedure (Step 806) and another symbol or codeword isdetermined in the same manner as above (Step 808).

Referring now to FIG. 9, a method for increasing data transfer capacityacross a network comprises receiving a signal composed of a plurality ofdata carrying sub-channels at a number of receiving elements (Step 900).A symbol or codeword associated with the signal received on thesub-channel at the receiving element is determined using a maximumlikelihood estimation process (Step 904). The determined symbol orcodeword is incorporated in the determination of at least one othersymbol or codeword (Step 906).

1. A method of determining each of a plurality of data symbols orcodewords from a plurality of signals comprising the steps of: (i)weighting a number of said signals so as to substantially null saidnumber of signals, using weighting apparatus; (ii) determining a symbolor codeword associated with at least one said, non-nulled signal using aprocessor arranged to execute a maximum likelihood estimation processupon said at least one, non-nulled signal; (iii) reducing the number ofsignals nulled by the weighting apparatus by at least a number ofnon-nulled signals in step (ii); (iv) altering a maximum likelihoodmetric in accordance with the data symbol derived at step (ii); and (iv)repeating steps (ii) to (iv).
 2. The method of claim 1 includingselecting signals with highest input power to be nulled during at leastone of steps (i) and step (iii).
 3. The method of claim 1 includingseparating frequencies of at least some of the plurality of signals bymultiples of a harmonic frequency.
 4. The method of claim 1 includingorthogonalising each of the plurality of signals.
 5. The method of claim1 including transmitting the plurality of signals at a range offrequencies from a plurality of spatially separated transmitters.
 6. Themethod of claim 1 including providing a plurality of receivers arrangedto receive said plurality of signals prior to step (i).
 7. The method ofclaim 1 including determining symbols that form at least part of,codewords, the codewords being associated with streams of symbols inputto a transmitter.
 8. The method of claim 1 including deriving a matrixof complex weighting coefficients by the processor to be applied to saidweighting apparatus in order to null said signals at one of step (i) andstep (iii).
 9. The method of claim 1 including sampling channel stateinformation data to determine which signals are to be nulled during atleast one of steps (i) and step (iii).
 10. The method of claim 1including using the vector Viterbi algorithm at step (ii).
 11. A signalreceiving apparatus comprising a plurality of receiving elements,weighting apparatus, and a decoder, each of the receiving elementshaving respective weighting apparatus associated therewith, each of thereceiving elements being arranged to receive a plurality of signalstransmitted from a plurality of transmitters, the weighting apparatusbeing arranged to apply a complex weighting function to each of a numberof said signals received by the receiving elements at a given frequencyin order to null said number of said signals, the decoder being arrangedto determine a symbol or codeword associated with a non-nulled signaland to incorporate said symbol or codeword in the determination of atleast one further symbol or codeword.
 12. Apparatus according to claim11 including at least four receiving elements.
 13. Apparatus accordingto claim 11 wherein each receiving element has a channel stateinformation (CSI) unit arranged to compensate for distortion to thesignal received by the apparatus due to variations in the transmissionpath of said signal associated therewith.
 14. Apparatus according toclaim 11 including an FFT unit arranged to separate each of a pluralityof sub-carrier signals from said received signals between each receivingelement and the decoder.
 15. Apparatus according to claim 11 wherein thedecoder includes a processor arranged to carry out a maximum likelihoodestimation procedure upon a sub-carrier signal received at a receivingelement in order to determine the symbol or codeword.
 16. Apparatusaccording to claim 15 wherein the processor is arranged to carry outwhole vector Viterbi decoding upon the signal.
 17. Apparatus accordingto claim 11 wherein the apparatus is arranged to execute a method ofdetermining each of a plurality of data symbols or codewords from aplurality of signals, said method comprising the steps of: (i) weightinga number of said signals so as to substantially null said number ofsignals, using weighting apparatus; (ii) determining a symbol orcodeword associated with at least one said non-nulled signal using aprocessor arranged to execute a maximum likelihood estimation processupon said at least one non-nulled signal; (iii) reducing the number ofsignals nulled by the weighting apparatus by at least a number ofnon-nulled signals in step (ii); (iv) altering a maximum likelihoodmetric in accordance with the data symbol derived at step (ii); and (v)repeating steps (ii) to (iv).
 18. A method of increasing data transfercapacity across a network comprising the steps of: (i) receiving asignal comprising a plurality of data carrying sub-channels at aplurality of receiving elements; (ii) suppressing a component of thesignal associated with a given sub-channel received at all but onereceiving element; (iii) determining a symbol or codeword associatedwith said signal received on said given sub-channel at said onereceiving element using a maximum likelihood estimation process; and(iv) incorporating the symbol or codeword into the maximum likelihoodestimation process for the determination of at least one other symbol orcodeword.
 19. The method of claim 18 including parallelising data andencoding the data as a symbol or a space time codeword prior totransmission of the symbol or codeword over the network.
 20. The methodof claim 18 including providing more than four receiving elementsarranged to receive the signal from the network.
 21. The method of claim18 including applying a whole vector Viterbi decoding to the signal atstep (iii).
 22. The method of claim 18 wherein the network is in theform of a wireless local area network or a mobile telecommunicationsnetwork.
 23. A computer readable medium having stored thereininstructions for causing a device to execute the method according toclaim
 1. 24. A program storage device readable by a machine and encodinga program of instructions which when operated upon the machine cause themachine to operate as the apparatus according to claim
 11. 25-27.(canceled)
 28. A computer readable medium having stored thereininstructions for causing a device to execute the method according toclaim 18.